Decide where you want to go.

Your plans will most likely change, but you should have a general idea so you can decide which city to fly to. When I planned the trip, I wanted to visit Berlin, Prague, Vienna, Budapest, Florence and Rome. Since this took me pretty much north to south, I opted for an open-jaw ticket that allowed me to fly into Berlin and out of Rome.

If your trip isn’t a circle, which makes flying in and out of the same city convenient, then I would highly recommend an open-jaw ticket. It doesn’t cost that much more than a round trip ticket and the difference is generally less than buying a one-way train ticket back to the city you started in. Who wants a 24 hour train ride right before flying 15 hours home?

I used Expedia because they made it easy to specify any number of arbitrary flights (and therefore, request an open-jaw ticket) and it found the cheapest fare for my specific request. If your entire trip lasts longer than a month (even by a day), your fare increases dramatically. At that point, you might as well stay for another month. When requesting fares from any of the services, it helps to try different start and end dates that are close to your preferred dates (as long as you are flexible). A change of one day can sometimes save you hundreds of dollars.